The invention relates generally to pulsed neutron instruments. More specifically, the present invention relates to monitoring source strength of a pulsed neutron source.
Determining the porosity and fluid content of subsurface earth formations are critical elements in maximizing the efficiency of oil, gas, and water (xe2x80x9cformation fluidsxe2x80x9d) exploration. To that end, a variety of techniques have been developed. One of the well known techniques involves irradiating the subsurface earth formations with high-energy neutrons and monitoring the resulting energy spectra. When neutrons bombard the formations surrounding the wellbore, they induce a radioactive response, generally in the form of neutrons and gamma radiation, which may be recorded by one or more detectors. Depending on the application, either or both types of radiation may be monitored. By using such techniques, it is possible to determine the porosity and fluid content of a given formation, which generally correspond to the amounts of various fluids that may be easily retrieved from a formation.
For neutron logging, the source used can be chemical or electrical in nature depending on the requirements of the application. The chemical neutron source has the advantage of being virtually indestructible. It has no electronic parts, so it can be relied upon to always produce neutrons (zero downtime). However, this is also a disadvantage of the chemical source. Because the emission of neutrons cannot be shut off, strict radioactive safety procedures must be followed. This inconvenience prompted the development of electronic neutron sources.
The advantage of an electronic neutron source (e.g., a Minitron(trademark) available from Schlumberger Technology Corporation (Sugar Land, Tex.)) is that it can be shut off, bringing the neutron emission levels to zero. This is both beneficial on the surface, where people are present, and downhole, in the event that the tool gets stuck and has to be abandoned. A Minitron(trademark) typically emits eight times as many neutrons with three times as much energy compared to a conventional chemical logging source. A Minitron(trademark) typically includes a ceramic tube containing tritium and deuterium at low pressure. This device creates neutrons at an energy of 14 MeV by accelerating deuterium ions into a tritium target. Such a system is often found in a pulsed neutron generator (PNG). When using such a pulsed neutron generator, the formation surrounding the well logging instrument is subjected to repeated, discrete xe2x80x9cburstsxe2x80x9d of neutrons. Being able to control the timing of bursts provides a pulsed neutron generator or an electronic neutron source a big advantage: more measurements are possible with an electronic neutron source than with a chemical neutron source because of the added time dimension.
Neutrons have no electric charge and their mass is similar to that of a proton. The lack of charge allows neutrons to penetrate into formations. This property of neutrons makes it ideal for logging applications. In the formation, neutrons interact with matter in a wide variety of ways. The characteristics of some of these interactions can be used to measure the formation properties.
Instruments that can make measurements for deriving various formation properties are described, for example, in U.S. Pat. No. 6,032,102 issued to Wijeyesekera et al., and in U.S. Pat. No. Re. 36,012 issued to Loomis et al., both assigned to the present assignee. Generally speaking, the instruments disclosed in these patents are arranged so that a pulsed neutron source therein emits a plurality of short duration neutron bursts. These bursts have a sufficient duration so as to enable relatively accurate measurement of density (through spectral analysis of inelastic gamma rays) and accurate measurement of porosity (through measurement of neutron count rates). A neutron detector positioned appropriately with respect to the source is used on such instruments to make the neutron count rate measurements. A gamma ray detector positioned appropriately with respect to the source, and coupled to a spectral analyzer, is used to make the inelastic gamma ray measurements. The short duration bursts are repeated for a selected number of times and the measurements made in appropriate time windows during and/or after each neutron burst are summed or stacked to improve the statistical precision of the measurements made therefrom. These instruments may also be adapted to measure neutron capture cross section of the earth formations.
In operating these neutron tools, it is often important to know the absolute strength of the nuclear source in order to calibrate the response of nuclear detectors. Pulsed neutron sources are used because of their enhanced safety compared to chemical sources and their ability to stimulate timing measurements. However, the output of a pulsed neutron source is prone to unpredictable and/or non-statistical changes over time. Therefore, it is desirable to have methods for monitoring the pulsed neutron source strength.
In principle, pulsed neutron source strength may be monitored by a detector having a high energy threshold such that it only detects high energy neutrons that have not interacted with the environment. Unfortunately, the count rates from such a detector are often too low to provide statistically reliable results under typical logging conditions. In addition, the stability of the high energy threshold may vary between different tools/detectors and over different temperatures. As a result, such count rates may not provide an accurate indication of the source strength under typical logging conditions. Thus, it is desirable to have better techniques for monitoring pulsed neutron source strength that can be used under typical logging conditions.
The invention provides a method for monitoring a pulsed neutron source. The method includes measuring a burst count rate while the pulsed neutron source is emitting neutrons using a monitor detector disposed proximate the pulsed neutron source; measuring a decay count rate while the pulsed neutron source is not emitting neutrons using the monitor detector; and deriving a source strength indicator by using the burst count rate and the decay count rate according to a selected function to substantially remove environmental effects in the burst count rate.
The invention provides a method for obtaining source strength-compensated measurements using a pulsed neutron tool equipped with a pulsed neutron source. The method includes measuring a burst count rate while the pulsed neutron source is emitting neutrons using a monitor detector disposed proximate the pulsed neutron source; measuring a decay count rate while the pulsed neutron source is not emitting neutrons using the monitor detector; deriving a source strength indicator from the burst count rate and the decay count rate; and using the source strength indicator to compensate measurements made by other detectors in the pulsed neutron tool for strength variations of the pulsed neutron source.
The invention provides a method for obtaining source strength-compensated measurements using a pulsed neutron tool equipped with a pulsed neutron source. The method comprises obtaining a first source strength indicator using a monitor detector disposed proximate the pulsed neutron source under controlled conditions; obtaining a second source strength indicator using the monitor detector under experimental conditions; obtaining an experimental count rate using a measurement detector under the experimental conditions; and compensating the experimental count rate for neutron source strength variation based on the first source strength indicator and the second source strength indicator.
The invention provides a pulsed neutron tool, including a tool body; a pulsed neutron source disposed in the tool body; a monitor detector disposed in the tool body proximate the pulsed neutron source; at least one measurement detector disposed in the tool body at a predetermined distance from the pulsed neutron source; and circuitry disposed in the tool body for controlling the pulsed neutron source, the monitor detector, and the at least one measurement detector.
Other aspects of the invention will become apparent from the following description, the drawings, and the claims.